The present invention relates to a catalyst for purifying exhaust gases of an automobile engine and a method of manufacturing the catalyst, for more effectively removing harmful gases from the exhaust gases of an automobile engine, such as hydrocarbons, carbon monoxide, and oxides of nitrogen (NO.sub.x).
Currently, with the increase in the number of automobiles, air pollution due to exhaust gases is becoming more and more serious. Diesel engine automobiles generate the most exhaust, and the chief harmful components of the exhaust are hydrocarbons, carbon monoxide, and oxides of nitrogen (NO.sub.x).
The research into methods of removing these harmful gases has been divided largely into fuel improvement, engine improvement, and exhaust treatment. In exhaust treatment, the use of a catalyst has proven to be most effective.
At present, the most typical catalyst for purifying the exhaust gases of an automobile engine is a 3-way catalyst in which a carrier coats a substrate, typically a honeycomb structure. The carrier is impregnated with a noble metal, such as platinum (Pt) or rhodium (Rh), and an oxygen absorbing material.
Such a 3-way catalyst shows a high degree of purification efficiency with respect to harmful 3-way gases, i.e., hydrocarbons, carbon monoxide, and oxides of nitrogen, at the ideal air-to-fuel ratio of 14.7:1. Recently, however, a lean-burn automobile engine has been developed in which combustion occurs in a lean-burn environment (an air-to-fuel ratio of 18.about.25:1), that is, with more air than the ideal air-to-fuel ratio.
Compared to the conventional automobile engine, the lean-burn automobile engine is advantageous in that the quantity of exhaust is decreased and fuel efficiency is improved. Although the lean-burn automobile engine has less exhaust, however, it is difficult to remove oxides of nitrogen from the exhaust, since in a lean-burn environment where the air-to-fuel ratio is high, oxidation of carbon monoxide and hydrocarbons readily occurs, but a reduction reaction is difficult. Thus, the lean-burn automobile engine has a particular problem in that oxides of nitrogen remain in the exhaust and act as a harmful pollutant.
Therefore, a catalyst capable of effectively removing oxides of nitrogen from the exhaust is required, especially for lean-burn automobile engines which operate in a lean-burn environment.
The catalysts for removing oxides of nitrogen can be largely divided into a selective contact reduction type and a storage reduction type. The selective contact reduction type catalyst wherein an active noble metal, such as Pt, Rh, or Pd, is impregnated into a Cu-ion exchange type zeolite, ZSM-5, is disadvantageous in that the removal efficiency for oxides of nitrogen is very low (about 35%) and heat-resistance and durability are poor.
Meanwhile, the storage reduction type catalyst in which a material absorbing oxides of nitrogen and an active noble metal, such as Pt, Rh, or Pd, are impregnated into alumina is advantageous in that heat-resistance and durability are good.
However, the conventional storage reduction type catalyst does not have a high purification efficiency for oxides of nitrogen because the absorbing material employed in the conventional catalyst does not have a high storage, i.e., absorbing, capacity for oxides of nitrogen.